Aeration apparatus are utilized in the treatment of water for the purpose of increasing the dissolved oxygen content of the water. A certain amount of dissolved oxygen is required for the life of fish and other aquatic organisms. Furthermore, dissolved oxygen is also required to prevent the formation of offensive odors and to break down organic matter in water. Therefore, aeration apparatus are especially useful in the biological purification of waste waters which contain organic solvents. It has also been found useful to increase the oxygen content of other substances, such as sludge.
Aeration apparatus used to increase the dissolved oxygen content of various substances are known. For example, U.S. Pat. No. 4,240,990 to Imhofer et al. and U.S. Pat. No. 4,280,911 to Durda et al. disclose motor driven propeller-type aerators of which the disclosures therein are hereby incorporated herein. However, such shaft powered aerators may be subjected to hostile conditions, such as temperatures below freezing. For example, if an aerator is shut off intentionally or unexpectedly due to temporary power loss, the turbulence caused by the aerator ceases and ice typically forms at the water surface and extends several inches therebelow due to the freezing temperatures and the lack of fluid flow. As a result, the aerator shaft may freeze up within the housing. Ice could also form immediately after shutdown at a location lower in the aerator assembly where the shaft bearing is positioned, if the aerator components were cooled below freezing due to colder ambient air and consequently colder air flowed through the aerator components before shutdown.
Thus, freezing temperatures may freeze the dynamic mechanisms of the aerator. As a result, those mechanisms may then undergo fatigue or even failure due to the torque of the motor when it is restarted. On the other hand, the frozen dynamic mechanisms could cause the motor to be overloaded. The resultant overamping of the motor would subject the motor to heat build-up which, if excessive, could permanently damage the motor windings. The damage may simply result in a reduction in the expected motor life or it may require replacement of the stator, rotor or related components of the motor. Since it is generally not cost effective to rewind a motor having less than 20 horsepower, replacement of the entire motor most likely would be necessary.
Aerator motors may include thermostatic protection devices therein to cut-off power input thereto in response to an increase in motor temperature, or include circuit breakers or fuses on their input side which would cut-off power input thereto in response to excessive current draw. However, these protective devices may prove ineffective and the motors may still overheat. These protective devices may be undersized due to error in design or inadvertent error during replacement thereof. Furthermore, such protective devices may fail to operate as intended due to latent defects therein, or failure in the contacts due to factors such as corrosion or the development of loose connections. Also, the above described protective devices may not respond to excessive current draw in time to prevent heat associated damage. Furthermore, even though such protective devices may provide a level of protection for the motor, after sensing an excessive current draw they must be replaced or reset before the motor may be restarted. Finally, these protective devices merely provide a cut-off function. They do not deice a frozen aerator, and therefore do not enable a frozen aerator to be started while the temperature remains below freezing. In fact, one would have to wait until the surrounding temperature rose above freezing and remained there long enough for the ice associated with the aerator to melt before using the aerator for its intended purpose.
Previous efforts to solve the above problems have included heating the aerator. Typically, the heating mechanisms used, due to their designs, had to be mounted outside of the aerator housing. Therefore, the externally provided heating mechanisms were located immediate to the heat sink, e.g., the ice, and radially separated from the dynamic mechanisms of the aerator by the entire aerator body. While affording simple mounting to aerator housings, heat transfer to the dynamic mechanisms of the aerator proved to be less than desirable or even ineffective.
One solution to this problem is described in commonly assigned, copending application Ser. No. 275,522, now U.S. Pat. No. 4,882,099. The application discloses an aspirator aerator including an outer tubular housing having an inner rotary driven member, such as a tube, disposed therein. A deicing mechanism is disposed between the inner tube and the outer tubular housing for deicing the aerator. This arrangement exhibits effective heat transfer characteristics and allows rapid deicing of the aerator under freezing, or even near arctic conditions, so as to prevent damage to the dynamic mechanisms of the aerator which are subject to the torque of the motor, and to prevent damage to the motor which enables the motor to remain in operation. While this arrangement is more effective than prior art heating mechanisms mounted outside the aerator housing, retrofitting the arrangement to existing aerators may prove difficult and requires the disassembly and reassembly of the aerator apparatus.